Authorisation

Fingers vs Fingerprints

It turned out that my "Authentication vs Identification" article was not sufficiently conclusive in the sense that some hardcore biometrics fans still nurture a non-trivial and well justified objection. So I need to address and destroy it, in order to close the topic. My opponents' argument is:

Your analysis narrows the both sides of the problem to a knowledge/ownership claim. Even if you are right, the conclusion is only applicable to the authentication by means of a knowledge token, whereas all the rest relations between the user and the token (suitable for authentication purposes) are set aside. There is one particularly important relation (the one fundamental for the entire biometrics field): «the user is» or other way around «the token is a part of the user» — this relation implies inalienability which makes the token safe for authentication purposes.

It is true. Completely true. It is undeniably true! In the physical realm.

The trick is that this relation indeed implies inalienability of the token, therefore it is not transferable. The very moment you scan my biometric data the relation (you base your authentication upon) is destroyed. A digital copy of my face is NOT a face and NOT mine either. Since the moment this copy is taken I have no control over it. The very reason of taking this copy was «my face belongs to me», but the copy does not! — you have lost the very relation you wanted to convey. This is how in the digital realm all relations between me an my digital assets reduce to the knowledge/ownership claims.

FOOTNOTE: Nor does this copy bear any information about other copies, nor does this copy bear any information about its creation or any fact of its past, where it has been, who have seen it, whether it is fake at all… Still you can identify this copy with my actual face, no doubt about it (the identification is not affected by the means you obtain this copy).

In the physical realm we have a whole bunch of tools to overcome this limitation. We use human witnesses as a representation of a very complex physical perception of the biometric data (although terribly faulty), we use physical world's limitations to assess feasibility of falsification the data, etc. All in all my fingerprints mean something to the police only when they are made of body grease placed on a relevant item and look reasonably «natural» — a sheet of paper with my fingerprints drawn in it has no value for a sane investigator.

— Hey, I have found and old photocopy of some fingerprints! This must be a suspect!
— WAT?!

It is that in the digital realm there is absolutely no difference between the «natural» fingerprints and the photocopies. This is the world of photocopies.

But, we have reliable procedures to deal with physical evidence, can we employ a machine to do the job? Yes, we can. What relation do we need to establish? «The token is the part of the user» or more specifically «the biometric data on the machine's sensors belongs to a living person». The trouble is, the machine perception is way too narrow. Presented with a human finger our machine can (in addition to scanning the picture) measure the temperature, humidity, electric conductivity… but none of these indicate a human being attached to this finger — a bag of salt could possess all these physical properties! EASILY!

In order to get some reliable readings we can create a very complex life monitor that connects to all my vital organs, or we can use an existing one — MY BRAIN (which is, by the way, the most reliable life monitor in the known universe — zero chance of false-positive life reports). Instead of connecting to my fingers, arms, legs, and other detachable parts of my body, our machine can connect directly to my brain, by means of English language. All we need to ensure the relation of bodily integrity between my biometric data and myself (I remind you, this relation is the foundation of the pro-biometrics argument) is to ask me straight:

Dear user, are you sure the finger on my fingerprint scanner is connected to your body?

«Gotcha!» — you may say — «how can you ensure the truthfulness of the answer?». In a very easy and natural manner: we can beforehand agree upon a secret obscure nondeductible answer, some sort of a code word… Oh! wait!..